Solar Charger to Ni-Cd Battery

Discussion in 'The Projects Forum' started by Phaisit, Feb 28, 2015.

  1. Phaisit

    Thread Starter New Member

    Feb 23, 2015
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    Hi, as I was trying to use buck converter before but it's really fail for me to use it (A lot of problem I faced and couldn't fix it)

    So I decide to change the circuit back to something basic but trying to add something.

    I want to use LM317 as Current Regulator to charging my 7.2V Battery
    And want to add some feedback here, when the voltage of battery reach maximum the Transistor will work as switch to stop charging
    (as picture I attach here)


    The calculation will be after this as I need some advice from you

    So I want to ask that do I miss something ? Should I add something for more good ?

    Thanks in advance
     
    Last edited: Feb 28, 2015
  2. wayneh

    Expert

    Sep 9, 2010
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    The LM317 can handle all the feedback. You don't need the op-amp.

    Are you thinking to use the op-amp as a voltage cut-off? You should read about charging strategies for your battery chemistry. Head over to Battery University for details. Is your battery really Ni-Cd as in your title, six cells at 1.2V?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Wayneh beat me to it. Turning off the charger when a certain final voltage is reached is not a valid method of charging NiCads or NiMh!
     
  4. Phaisit

    Thread Starter New Member

    Feb 23, 2015
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    It's a 7.2V 2800mAh Ni-Cd battery for a RC Toy.
    I just want to add some feedback maybe will add some LED to show the status that it's charging or not.
    It's kind of my project so I just need to add something to make it look more better
     
  5. Dodgydave

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    Jun 22, 2012
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    what current are you charging the battery with,you may be able to put a Led in series with the output if its under 20mA,

    Or you can put a led with a 100R resistor across the In and Adj terminals, then when the battery is connected the led lights.


    http://www.stufinnis.co.uk/aacharger.html
     
    Last edited: Feb 28, 2015
  6. wayneh

    Expert

    Sep 9, 2010
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    I like the LM339 quad comparator for this sort of thing. You can use it indicate that a connection is made, that charging is occurring, maybe even at different levels, and so on.

    But you still need to follow a valid charging strategy.
     
  7. Phaisit

    Thread Starter New Member

    Feb 23, 2015
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    So it's mean that I don't switch off when the battery is full ? So what can I do to make it look more difficult ?
    I have an idea about using Arduino to show the status of battery but didn't know if it will work.

    I want to charge with around 500mA. and want to build like Green led to show that it's charging and Red led to show it stop charging something like this
     
  8. Dodgydave

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    Jun 22, 2012
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    Ok, i would use a window comparator to set the upper and lower charge voltages, you can use an Lm358 or a 555 timer in comparator mode.
     
  9. MikeML

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    This still doesn't solve the fundamental problem: Read my lips:
    Using a rise in battery voltage is not a valid method of detecting the end-of-charge for NiCads or NiMh!
     
  10. Phaisit

    Thread Starter New Member

    Feb 23, 2015
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    So I have to discharge the battery to the minimum first before charging right ?
     
  11. MikeML

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    not necessarily. Unfortunately, detecting end-of-charge for NiCds is problematic. Read about suitable strategies here.
     
  12. Phaisit

    Thread Starter New Member

    Feb 23, 2015
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    OK, so if I change to the method for CC/CV should I use LM317 as current limiter for CC mode and when the Voltage is in the right amount I will switch it to a CV mode is it acceptable ? for a flood charging method
     
  13. Dodgydave

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    Jun 22, 2012
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    WEll it works for me, never had a problem before with nicads,if your going to start monitoring temperature,voltage levels, then you might as well buy a ready made nicad/nimhi charger.
     
    Last edited: Feb 28, 2015
  14. MikeML

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    It is likely that the NiCds will accept a charge rate much higher than what your panel delivers, so no current-limiter is even needed.

    CV (i.e. a shunt regulator) is not a valid method of terminating charging. What would be the correct voltage? I defy you to find one!

    Millions of NiCd battery packs have been sold that include a thermistor inside the pack, and use temperature rise to shut off the charger. If you cannot do that, read the Battery University Page very carefully. Here is another source I trust that talks about charging NiCds.
     
  15. Phaisit

    Thread Starter New Member

    Feb 23, 2015
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    Can I see your charger schematic ?
     
  16. wayneh

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    That's the "solution" used in just about every commercial device I've ever seen. The battery just sits there taking whatever the panel can make.

    A SLA battery system I have includes a voltage-based shunt to send excess current to ground once the SLA battery reaches the target voltage. That's valid for lead-acid but I've never seen it in a NiCd system. I think the fancier NiCd chargers can shorten charging time by delivering a higher current when the battery voltage is low and then go to a lower current as the voltage comes up, holding that trickle rate forever.
     
  17. Dodgydave

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    Jun 22, 2012
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    here is the circuit i use for nicads.

    nicad.png
     
  18. MikeML

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    Which is nothing but a current limiter, with no end-of-charge detection. If you leave it connected too long, you cook the NiCads. How long do you leave it connected????
     
  19. Dodgydave

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    i charge it at 10% of their value for ten hours, for fast charge i use 1 amp for 1hour.
     
  20. MikeML

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    Dave, I not trying to be argumentative, but I am attempting to point out that with your method of using time to disconnect the NiCds, you must know three things:
    1.The state if charge of the batteries before you start.
    2. The actual charging current.
    3. Keep track of time.

    If you are starting with a solar panel, then you only guessing at the charging current. You are always guessing at the state of charge before you start. Amout the only thing you can be sure of is time, and only if you remember to disconnect the charger at the right time.
     
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